Powder-coated substrates with topcoat based on silanes containing epoxide groups

ABSTRACT

The invention relates to powder-coated substrates bearing on a powder-coated surface a scratch-resistant and abrasion-resistant topcoat of a coating material which comprises: 
     a) condensates based on hydrolyzable silanes containing at least one non-hydrolyzable substituent, the hydrolyzable silanes having an epoxide group on at least one non-hydrolyzable substituent; 
     b) a curing catalyst selected from Lewis bases and alkoxides of titanium, zirconium or aluminium; 
     c) nanoscale particulate inorganic solids having a particle size in the range from 1 to 100 nm; and 
     d) at least one organic monomer, oligomer or polymer containing at least one epoxide group.

The present invention relates to powder-coated substrates with ascratch-resistant and abrasion-resistant topcoat of a coating materialcomprising condensates based on hydrolysable silanes containing at leastone epoxide group, and processes for producing the powder-coatedsubstrates.

The advantages of powder coating materials for the coating of surfaces,such as very good adhesion to metal surfaces, corrosion protection formetal surfaces, and ease of processability, for example, are known fromthe prior art. However, there is great interest in improvements to thecoat morphology (increase in smoothness and uniformity, reduction incoat thickness) and in increasing the surface hardness and abrasionresistance. In the field of acrylate powder coating materials, a numberof processes are known for providing the powder-coated surface with acoating based on organic-inorganic copolymers which is applied by wetchemical means.

JP-A-04318088 describes a methacryloyloxy-propyltrimethoxysilane-styrenecopolymer for coating surfaces coated with an acrylate powder coatingmaterial. The transparent coatings, 20-30 μm in thickness, are said topossess good acid-resistance and scratch-resistance.

JP-A-06039349 describes a coating material based on polymerizationproducts of hydrolysable silanes with a methacryloyloxypropylsubstituent, further hydrolysable silanes, acrylate, methacrylate andepoxy methacrylate, and also the curing catalyst aluminiumtris(acetylacetonate).

It is known that essentially inorganic coatings, i.e. coatings based oninorganic components, frequently have good surface hardness and abrasionresistance. Because of their high hardness, however, inorganic coatingsare more brittle than essentially organic coatings, i.e. coatings basedon organic components, with the result that cracking may occur. Inparticular, inorganic coatings are more brittle than organic powdercoatings, with the result that cracking may occur on thermal curing attemperatures near the crosslinking temperature of the powder coatingmaterial and on cyclic temperature loading of the composite coatingsystem.

The present invention is therefore based on the object of applying athin, abrasion-resistant coat to powder-coated surfaces without theoccurrence of cracking or brittleness under temperature load.

This object is achieved by means of the powder-coated substrates of theinvention, bearing on a powder-coated surface a scratch-resistant andabrasion-resistant topcoat of a coating material which comprises:

a) condensates based on hydrolysable silanes containing at least onenon-hydrolysable. substituent, the hydrolysable silanes having anepoxide group on at least one non-hydrolysable substituent;

b) a curing catalyst selected from Lewis bases and alkoxides oftitanium, zirconium or aluminium;

c) nanoscale particulate inorganic solids having a particle size in therange from 1 to 100 nm; and

d) at least one organic monomer, oligomer or polymer containing at leastone epoxide group.

The invention further provides a process for producing a coatedsubstrate, in which a powder coating material is applied to and cured onan optionally precoated substrate and on the powder coating a topcoatmaterial comprising the abovementioned components a) to d) is appliedand cured.

In accordance with the invention, surprisingly thin, highlyabrasion-resistant coatings are obtained which adhere particularly wellto powder-coated surfaces, are very well adapted to the powder coatingin their flexibility, and consequently possess a markedly improvedcyclic temperature resistance (no cracking during preparation andapplication) and, moreover, exhibit very good scratch resistance orsurface hardness and abrasion resistance.

In relation to the prior art, the process of the invention is easier tocarry out, since it does not require complex preparation of copolymersor polymerization products of hydrolysed silanes with organic monomersor oligomers prior to the application of the coating material, so thatit is possible to form substantially thinner coats having theaforementioned advantages.

The hydrolysable silanes containing at least one non-hydrolysablesubstituent, the hydrolysable silanes having an epoxide group on atleast one non-hydrolysable substituent, comprise one or more siliconcompounds possessing from 1 to 3, preferably 2 or 3, with particularpreference 3, hydrolysable radicals and 1, 2 or 3, preferably one,non-hydrolysable radical. At least one of the non-hydrolysable radicalspossesses at least one epoxide ring.

The silanes of component a) comprise, for example, compounds of thegeneral formula (I):

R_(n)SiX_(4−n)  (I)

in which n is 1, 2 or 3, preferably 1 or 2, with particular preference1, X may be identical or different and is a halogen (F, Cl, Br and I,especially Cl and Br), alkoxy (especially C₁₋₄ alkoxy, such as methoxy,ethoxy, n-propoxy, isopropoxy and butoxy, for example), aryloxy(especially C₆₋₁₀ aryloxy, e.g. phenoxy), acyloxy (especially C₁₋₄acyloxy, such as acetoxy and propionyloxy, for example) andalkylcarbonyl (e.g. acetyl), and R may be identical or different and isa non-hydrolysable radical, at least one radical R having an epoxidegroup.

Particularly preferred hydrolysable radicals X are alkoxy groups,especially methoxy and ethoxy. Examples of non-hydrolysable radicals Rwithout an epoxide ring are alkyl, especially C₁₋₄ alkyl (such asmethyl, ethyl, propyl and butyl, for example), alkenyl (especially C₂₋₄alkenyl, such as vinyl, 1-propenyl, 2-propenyl and butenyl, forexample), alkynyl (especially C₂₋₄ alkynyl, such as acetylenyl andpropargyl, for example) and aryl (especially C₆₋₁₀ aryl, such as phenyland naphthyl, for example), it being possible for the groups justmentioned to carry, if desired, one or more substituents, such ashalogen and alkoxy, for example. Also deserving of mention in thiscontext of radicals R are methacrylic and methacryloxypropyl radicals.

Examples of non-hydrolysable radicals R with an epoxide ring are inparticular those possessing a glycidyl or glycidyloxy group. They may belinked to the silicon atom via an alkylene group, e.g. a C₁₋₆ alkylene,such as methylene, ethylene, propylene, butylene. Specific examples ofhydrolysable silanes which may be used in accordance with the inventionmay be found, for example, in EP-A-195493.

Hydrolysable silanes which are particularly preferred in accordance withthe invention are those of the general formula (II):

X₃SiR  (II)

in which the radicals X, identical to or different from one another(preferably identical), are a hydrolysable group, for example one of theradicals X described above for the formula (I), preferably C₁₋₄ alkoxyand with particular preference methoxy and ethoxy, and R is aglycidyloxy-C₁₋₆ alkylene radical. Owing to its ready availability,γ-glycidyloxypropyltrimethoxysilane (referred to below as GPTS forshort) is used with particular preference in accordance with theinvention.

In addition to the hydrolysable silanes containing at least one epoxidegroup, other hydrolysable compounds may also be used to construct theinorganic matrix. By other hydrolysable compounds are meant hereinbelowthose not comprising hydrolysable silane containing at least one epoxidegroup. These other compounds likewise include an inorganic element withhydrolysable substituents attached to it.

It is possible to use one or more other hydrolysable compounds togetherwith the hydrolysable silane or silanes containing at least one epoxidegroup in component a), the amount of the other hydrolysable compoundspreferably not exceeding 80 mol %, and especially 60 mol %, based oh thetotal hydrolysable compounds employed. Preferably at least 10, and withparticular preference at least 20, mol % of all hydrolysable compoundsemployed are the other hydrolysable compounds which are different fromthe hydrolysable silane or silanes containing at least one epoxide groupon a non-hydrolysable substituent.

Examples of suitable other hydrolysable compounds are hydrolysablecompounds of elements selected from the third and fourth main groups(especially B, Al, Ga, Si, Ge and Sn) and from the third to fifthtransition groups of the Periodic Table (especially Ti, Zr, Hf, V, Nband Ta). However, other metal compounds may also lead to advantageousresults, such as those of Zn, Mo and W, for example. With particularpreference, the compounds in question comprise hydrolysable compounds ofelements from the group consisting of Si, Ti, Zr, Al, B, Sn and V whichare hydrolysed with the hydrolysable silane or silanes of component a).

All of these compounds contain hydrolysable groups. As examples,reference may be made to the examples of X listed for formula (I). Inaddition to the hydrolysable groups, the compounds may also containnon-hydrolysable groups. Except for Si, however, this is not preferred.As examples, reference may be made to the examples of R listed forformula (I), with the proviso that R is not an epoxy-containing group.For the silanes which may be used, reference may be made—with theaforementioned proviso—for example to the general formula (I), in whichn may also be 0. Specific examples of these other hydrolysable compoundsare:

Si(OCH₃), Si(OC₂H₅)₄, Si(O-n- or i-C₃H₇)₄, Si (OC₄H₉)₄, SiCl₄, HSiCl₃,Si(OOCCH₃)₄, CH₃—SiCl₃, CH₃—Si(OC₂H₅)₃, C₂H₅—SiCl₃, C₂H₅—Si(OC₂H₅)₃,C₃H₇—Si(OCH₃)₃, C₆H₅—Si(OCH₃)₃, C₆H₅—Si(OC₂H₅)₃, (CH₃O)₃—Si—C₃H₆—Cl,(CH₃)₂SiCl₂, (CH₃)₂Si(OCH₃)₂, (CH₃)₂Si(OC₂H₅)₂, (CH₃)₂Si(OH)₂,(C₆H₅)₂SiCl₂, (C₆H₅)₂Si(OCH₃)₂, (C₆H₅)₂Si(OC₂H₅)₂, (i-C₃H₇)₃SiOH,CH₂═CH—Si(OOCCH₃)₃, CH₂═CH—SiCl₃, CH₂═CH—Si(OCH₃)₃, CH₂═CH—Si(OC₂H₅)₃,CH₂═CH—Si(OC₂H₄OCH₃)₃, CH₂═CH—CH₂—Si(OCH₃)₃, CH₂═CH—CH_(2—Si(OC) ₂H₅)₃,CH₂═CH—CH₂—Si(OOCH₃)₃, CH₂═C(CH₃)—COO—C₃H₇—Si(OCH₃)₃,CH₂═C(CH₃)—COO—C₃H₇—Si(OC₂H₅)₃, Al(OCH₃)₃, Al(OC₂H₅)₃, Al(O-n-C₃H₇)₃,Al(O-i-C₃H₇)₃, Al(OC₄H₉)₃, Al(O-i-C₄H₉)₃, Al(O-sec-C₄H₉)₃, AlCl₃,AlCl(OH)₂, Al(OC₂H₄OC₄H₉)₃, TiCl₄, Ti(OC₂H₅)₄, Ti(OC₃H₇)₄,Ti(O-i-C₃H₇)₄, Ti(OC₄H₉)₄, Ti(2-ethylhexoxy) 4; ZrCl₄, Zr(OC₂H₅)₄,Zr(OC₃H₇)₄, Zr(O-i-C₃H₇)₄, Zr(OC₄H₉)₄, ZrOCl₂, Zr(2-ethylhexoxy)₄, andalso Zr compounds containing complexing radicals, such as β-diketone andmethacrylic radicals, for example,

BCl₃, B(OCH₃)₃, B(OC₂H₅)₃, SnCl₄, Sn(OCH₃)₄, Sn(OC₂H₅)₄, VOCl₃ andVO(OCH₃)₃.

As can be seen, these compounds (especially the silicon compounds) mayalso possess non-hydrolysable radicals having a carbon-carbon double ortriple bond. Where such compounds are used together with the siliconcompound of component a), it is also possible in addition to incorporate(preferably hydroxyl-containing) unsaturated monomers, such as(meth)acrylates, for example, into the composition. In the case ofthermal or photochemically induced curing of the correspondingcompositions, then, in addition to the build-up of the organicallymodified inorganic matrix, an addition polymerization of the speciescontaining unsaturated bonds takes place, thereby raising thecrosslinking density and thus also the hardness of the correspondingcoatings.

Furthermore, it is also possible in addition or alone to use, as otherhydrolysable compounds, for example, one or more hydrolysable siliconcompounds having at least one non-hydrolysable radical having from 5 to30 fluorine atoms attached to carbon atoms which if desired areseparated from Si by at least two atoms. Hydrolysable groups which maybe used in this context are, for example, those as specified for X informula (I). The use of a fluorinated silane of this kind additionallygives the coating in question hydrophobic and oleophobic(dirt-repelling) properties.

The hydrolysable fluorinated silanes which may additionally be used inthe topcoat material are those which possess at least onenon-hydrolysable radical having from 5 to 30 fluorine atoms attached tocarbon atoms which if desired are separated from Si by at least twoatoms. Silanes of this kind are described in detail in DE 4118 184.Specific examples thereof are the following:

C₂F₅CH₂—CH₂—SiY₃

n-C₆F₁₃CH₂CH₂—SiY₃

n-C₈F₁₇CH₂CH₂—SiY₃

n-C₁₀F₂₁CH₂CH₂—SiY₃

(Y═OCH₃, OC₂H₅ or Cl)

i-C₃F₇O—(CH₂)₃—SiCl₂(CH₃)

n-C₆F₁₃CH₂CH₂SiCl₂(CH₃)

n-C₆F₁₃CH₂CH₂SiCl(CH₃)₂

These fluorinated silanes are used in general in an amount from 0.1 to15, preferably from 0.2 to 10 and with particular preference from 0.5 to5% by weight, based on the mass of all hydrolysable compounds (componenta) and the other hydrolysable compounds) and of components b) to d).

The hydrolysable silanes of component a) and, if appropriate, the otherhydrolysable compounds are used together, for example, in an amount offrom 40 to 90% by weight, based on the mass of all hydrolysablecompounds and of components b) to d).

The coating material for the topcoat further comprises a curing catalystselected from Lewis bases and alkoxides of titanium, zirconium oraluminium. This curing catalyst acts in particular as a catalyst forepoxide-epoxide and/or polyol-epoxide crosslinking. In the correspondingcompositions, the curing catalyst is used in general in an amount offrom 0.01 to 0.6 mol per mole of epoxide group of the hydrolysablesilanes of component a). Preference is given to amounts in the rangefrom 0.02 to 0.4, and in particular from 0.05 to 0.3, mol of curingcatalyst per mole of epoxide group.

As the curing catalyst it is possible, for example, to use a Lewis base.The Lewis base preferably comprises a nitrogen compound. Nitrogencompounds of this kind may be selected, for example, fromN-heterocycles, amino-containing phenol's, polycyclic amines and ammonia(preferably in the form of an aqueous solution). Specific examples are1-methylimidazole, 2-(N,N-dimethylaminomethyl)phenol,2,4,6-tris(N,N-dimethylaminomethyl)phenol and1,8-diazabicyclo[5.4.0]-7-undecene. Among these compounds, particularpreference is given to 1-methylimidazole.

A further class of nitrogen-containing Lewis bases which may be used inaccordance with the invention are hydrolysable silanes possessing atleast one non-hydrolysable radical comprising at least one primary,secondary or tertiary amino group. Such silanes may be hydrolysedtogether with the hydrolysable silane or silanes of component a) and inthat case constitute a Lewis base built into the organically modifiedinorganic network. Preferred nitrogen-containing silicon compounds arethose of the general formula (III):

X₃SiR″  (III)

in which the radicals X are defined as above in the case, _(o)f thegeneral formula (I) and R″ is a non-hydrolysable radical attached to Siand comprising at least one primary, secondary or tertiary amino group.Specific examples of silanes of this kind are3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane,N-(2-aminoethyl)-3-aminopropyltrimethoxysilane,N-[N′-(2′-aminoethyl)-2-aminoethyl]-3-aminopropyltrimethoxysilane andN-[3-(triethoxysilyl)propyl]-4,5-dihydroimidazole.

Instead of or in addition to the Lewis base it is possible as curingcatalyst to use an alkoxide of Ti, Zr or Al. The alkoxide in question ispreferably of the general formula (IV):

M(OR′″)_(m)  (IV)

in which M is Ti, Zr or Al, R′″ is an alkyl group having preferably 1 to4 carbon atoms (methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butylor tert-butyl) or an alkyleneoxyalkyl group having preferably 1 to 4carbon atoms both for the alkylene unit and for the alkyl unit (e.g.methylene, ethylene, 1,2-propylene, 1,3-propylene and 1,4-butylene forthe alkylene unit and the examples given above for the alkyl group forthe alkyl unit) and n is 4 (M=Ti, Zr) or 3 (M=Al). Preferred curingcatalysts are Al(OCH₂CH₂OC₄H₉)₃ (aluminium tributoxyethoxide), where thebutyl group is preferably an n-butyl group, aluminium sec-butoxide, andmixtures of aluminium tributoxy ethoxide and aluminium sec-butoxide.

The curing catalyst is used, for example, in an amount of from 2 to 15%by weight, based on the mass of all hydrolysable compounds and ofcomponents b) to d).

The nanoscale particulate inorganic solids may comprise any desiredinorganic materials, but in particular comprise metal or metal compoundssuch as, for example, (optionally hydrated) oxides such as ZnO, CdO,SiO₂, TiO₂, ZrO₂, CeO₂, SnO₂, Al₂O₃, In₂O₃, La₂O₃, Fe₂O₃, Cu₂O, Ta₂O₅,Nb₂O₅, V₂O₅, MoO₃ or Wo₃; chalcogenides such as, for example, sulphides(e.g. CdS, ZnS, PbS and Ag₂S), selenides (e.g. GaSe, CdSe and ZnSe) andtellurides (e.g. ZnTe or CdTe), halides such as AgCl, AgBr, AgI, CuCl,CuBr, CdI₂ and PbI₂; carbides such as CdC₂ or SiC: arsenides such asAlAs, GaAs and GeAs; antimonides such as InSb; nitrides such as BN, AlN,Si₃N₄ and Ti₃N₄; phosphides such as GaP, InP, Zn₃P₂ and Cd₃P₂;phosphates, silicates, zirconates, aluminates, stannates and thecorresponding mixed oxides (e.g. those with perovskite structure such asBaTiO₃ and PbTiO₃). It is possible to use one kind of nanoscaleparticulate inorganic solids or a mixture of different nanoscaleparticulate inorganic solids.

The nanoscale particulate inorganic solids preferably comprise an oxide,oxide hydrate, nitride or carbide of Si, Al, B, Zn, Cd, Ti, Zr, Ce, Sn,In, La, Fe, Cu, Ta, Nb, V, Mo or W, with particular preference of Si,Al, B, Ti and Zr. Preferred particulate materials are boehmite, ZrO₂ andTiO₂ and also titanium nitride. Particular preference is given tonanoscale boehmite particles.

The nanoscale particulate inorganic solids are obtainable commerciallyin the form of powders and the preparation of (acidically stabilized)sols thereof is likewise known in the prior art. Furthermore, referencemay also be made for this purpose to the preparation examples givenbelow. The principle of the stabilization of nanoscale titanium nitrideby means of guanidine propionic acid is described, for example, in DE 4334 639.

The variation of the nanoscale particles generally goes hand in handwith a variation of the refractive index of the corresponding materials.Thus, for example, replacing boehmite particles by ZrO₂ or TiO₂particles leads to materials having higher refractive indices, therefractive index resulting additively, in accordance with theLorentz-Lorenz equation, from the volume of the component of highrefractive index and the volume of the matrix.

The nanoscale particulate inorganic solids generally possess a particlesize in the range from 1 to 100 nm, preferably from 2 to 50 nm, and withparticular preference from 5 to 20 nm. This material may be used in theform of a powder but is preferably used in the form of a sol (especiallyan acidically stabilized sol).

The nanoscale particulate inorganic solids may be used, especially whenvalue is placed on very good properties of high scratch resistance, inan amount of up to 50% by weight, based on the mass of all hydrolysablecompounds and of components b) to d). In general, the amount ofnanoscale particulate inorganic solids is in the range from 1 to 40,preferably from 1 to 30, with particular preference from 1 to 15% byweight, based on the mass of all hydrolysable compounds and ofcomponents b) to d).

It is also possible to use nanoscale particulate inorganic solids whichhave been provided with addition-polymerizable and/or polycondensableorganic surface groups. Such addition-polymerizable and/orpolycondensable nanoparticles and their preparation are described, forexample, in DE 19746885. It is also possible to use nanoscaleparticulate inorganic solids which on the basis of Lewis base groups orLewis acid groups on the particle surface also act as a hydrolysis andcuring catalyst. Nanoscale particulate solids modified in this way aredescribed in PCT/EP98/03846.

As a further component, the topcoat material used in accordance with theinvention comprises at least one organic monomer, oligomer or polymercontaining at least one epoxide group, or mixtures thereof. Theseorganic monomers, oligomers or polymers containing epoxide groups are,for example, compounds which are known per se and which are used inaccordance with the prior art as epoxy resins, casting resins and epoxyreactive diluents. They may comprise, for example, aliphatic,cycloaliphatic or aromatic compounds, aliphatic, cycloaliphatic oraromatic esters or ethers, or mixtures thereof, based for example onethylene glycol, 1,4-butanediol, propylene glycol, 1,6-hexanediol,cyclohexanedimethanol, pentaerythritol, bisphenol A, bisphenol F orglycerol, in each case as monomers, oligomers or polymers, which containat least one epoxide group. They may also contain a plurality of epoxidegroups; in the case of monomers or oligomers, for example, 2 or 3.

Specific examples are 3,4-epoxycyclohexylmethyl3,4-epoxycyclohexanecarboxylate, bis(3,4-epoxycyclohexyl) adipate,1,4-butanediol glycidyl ether, phenyl glycidyl ether, o-cresyl glycidylether, p-tert-butylphenyl glycidyl ether, cyclohexanedimethanoldiglycidyl ether, glycerol triglycidyl ether, neopentyl glycoldiglycidyl ether, pentaerythritol polyglycidyl ether, 2-ethylhexylglycidyl ether, 1,6-hexanediol diglycidyl ether, polypropylene glycoldiglycidyl ether, epoxy resins based on bisphenol A, epoxy resins basedon bisphenol F, and epoxy resins based on bisphenol A/F.

The epoxy resins which may be used preferably have an epoxide equivalentweight of from 130 to 455 g/equivalent and are preferably liquid with aviscosity of from 1.2 to 12,000 mPas at 25° C.

As organic monomer, oligomer or polymer containing at least one epoxidegroup it is possible, for example, to use readily available commercialproducts, such as the products Ruetapox® (from Bakelite AG), theproducts Polypox R® (from U. Prümmer Polymer-Chemie GmbH), the productsAraldit® GY 257, Araldit® GY 266, Araldit GY 179, Araldit® PY 309,Araldit® DY 3601, Araldit® CIBA, Araldit® GY 285 (from Carl Roth andfrom Ciba-Geigy) and the products Cyracure® Resin UVR 6128, Cyracure®Resin UVR 6110 (from Union Carbide).

The organic monomer, oligomer or polymer containing at least one epoxidegroup is used, for example, in an amount of from 6 to 30% by weight,based on the mass of all hydrolysable compounds and of components b) tod).

To form the condensates a), the hydrolysable silanes having at least onenon-hydrolysable substituent containing at least one epoxide group,together if used with the above-described other hydrolysable compounds,are subjected to hydrolysis. Hydrolysis is preferably carried out usinga catalyst. Preferred catalysts are those which are not at the same timecondensation catalysts for the epoxide-epoxide crosslinking. A preferredacidic catalyst is aqueous HCl. The hydrolysis is preferably conductedusing from 0.5 to 4.0 mol of H₂O per mole of hydrolysable radical. Thehydrolysis is conducted, for example, at room temperature.

The hydrolysis is also accompanied by condensation reactions between thehydrolysable compounds, producing condensates. The degree ofcondensation depends on the reaction parameters, so that the personskilled in the art can adjust the degree of condensation, as and whenrequired, by adjusting these parameters. In general, the procedure issuch that, although some condensation has taken place prior to theaddition of the remaining components b) to c), condensation is not yetcomplete, so that it is also possible to talk of precondensates.However, the nanoscale particulate inorganic solids (component c)) mayalso be added to the hydrolysable silanes and, if appropriate, the otherhydrolysable compounds prior to the hydrolysis. Preferably, thenanoscale particulate inorganic solids are added in the form of asuspension in the preferred catalyst, aqueous HCl.

Hydrolysis and partial condensation are followed by the addition of theother components in any desired sequence. As mentioned, however, thehydrolysis may also take place, for example, in the presence of thenanoscale particulate inorganic solids.

Where in addition to the relatively slow-to-react silicon compounds useis also made of other hydrolyzable compounds which are more reactive,such as compounds of Ti, Zr and Al, for example, it is advisable to addthe water in steps and/or with ice cooling and/or to use compounds whichhave been made slower to react by complexation (as in the case ofAl(OC₂H₄OC₄H₉)₃, for example).

If desired, inert solvents may be added to the compositions at any stageof the preparation for the purpose of adjusting the rheologicalproperties of the compositions. These solvents preferably comprisealcohols and/or alcohol ethers which are liquid at room temperature, forexample C₁₋₈ alcohols, which in addition are also formed during thehydrolysis of the preferably employed alkoxides of the correspondingelements, or monoethers of diols such as ethylene glycol or propyleneglycol with C₁₋₈ alcohols.

Furthermore, the coating material for the topcoat may comprise furtheradditives. These comprise the customary additives, such as colorants,levelling agents, UV stabilizers, antioxidants, such as stericallyhindered amines (HALS), photoinitiators, photosensitizers (ifphotochemical curing of the composition is intended), and thermalpolymerization catalysts, for example.

The substrate to be coated may comprise, for example, a substrate madeof metal, glass, plastic, wood, or ceramic. The substrate is preferablyof metal. The substrate may have been pretreated, by phosphating, forexample. If appropriate, the substrate has already been provided withcustomary primers or coatings.

A powder coating material is applied to the substrate. A powder coatingmaterial comprises a coating material which is applied in powder formand whose film-forming phase comprises binder, possibly pigments,possibly fillers and possibly additives, and which after baking forms acoating film. The powder coating material employed may comprise any ofthe powder coating materials known from the prior art. For thermosetcoating films (thermosetting powder coatings), curing agents and binderssuch as epoxides, polyesters, epoxy-polyester mixtures, polyurethanes oracrylates, for example, are used. In the case of thermoplastic powdercoating materials, polyolefins or PVC, for example, are used as binders.

The powder coating material is applied by the customary techniques.Subsequently, a film on the substrate is formed from the powder, and iscured. The coating material for the topcoat is then applied to the curedcoating film formed from the powder coating material.

The application of the above-described topcoat material to thepowder-coated substrate may take place by standard coating techniques,such as dipping, spreading, brushing, knife coating, rolling, sprayingand spin coating, for example.

Subsequently, directly or after initial drying at room temperaturebeforehand (for partial removal of the solvents), a curing(condensation) is carried out. Curing preferably takes place thermallyat temperatures in the range from 50 to 300° C., preferably from 70 to200° C., with very particular preference from 90 to 180° C., undereduced pressure if desired.

By means of the topcoat materials used in accordance with the invention,topcoat thicknesses of, for example, from 1 to 30 μm, preferably from 1to 20 μm and in particular from 3 to 10 μm may be obtained.

The coatings may if desired possess high transparency and are alsonotable for high scratch resistance, long-term hydrophilicity (as aresult of the acidically catalyzed hydrolysis) and dirt-repellingproperties (if fluorinated silanes are used in addition). If certainsurfactants are added to the composition of the topcoat material, animprovement in the (long-term) hydrophilicity is obtained. If certainaromatic polyols are added to the composition of the topcoat material,the coatings obtained are particularly corrosion-inhibiting. Forappropriate surfactants and aromatic polyols, reference may be made toWO 95/13326.

Furthermore, it has surprisingly been found that by using the organicmonomers, oligomers and/or polymers containing at least one epoxidegroup it is possible to obtain no disadvantageous embrittlement butrather, in contrast, an advantageous flexibilization without adverselyaffecting the scratch-, abrasion- or acid-resistance. Very well-adheringtopcoat films are formed whose flexibility is very well adapted to thepowder coating and which therefore possess a markedly improved cyclictemperature stability (no cracking during preparation and application).The topcoat compensates for unevennesses in the powder coating surface(orange peel), and the coated particles acquire a more appealingappearance.

The topcoat materials used in accordance with the invention aretherefore outstandingly suitable for coating powder-coated substrates.They are particularly suitable, for example, for coating powder-coatedsurfaces or casings, especially metal casings, of industrial equipmentor powder-coated surfaces of furniture, iron goods and means oftransport.

Examples of powder-coated surfaces or casings, especially metal casings,of industrial equipment are industrial ovens, switching desks, computersand production plants. Examples of powder-coated surfaces of furnitureare surfaces of shelving, cupboards, chairs and tables. Examples ofpowder-coated surfaces of iron goods are surfaces of frames of windowsand doors, valves, fittings, railings and posts. Examples ofpowder-coated surfaces of means of transport are surfaces of bicycleframes, mopeds and motorbikes, bodywork parts and wheel rims of motorvehicles.

The examples which follow are intended to illustrate the presentinvention without, however, restricting its scope.

EXAMPLE 1

a) Preparation of a Boehmite Sol

104.62 g of 0.1 N HCl were added to 12.82 g of acetic acid-stabilized(6.4% by weight acetic acid) boehmite powder. Subsequent ultrasoundtreatment (20 minutes) produced a transparent, colourless solution whichwas used further directly to prepare the coating sol.

b) Preparation of the Coating Sol

24.3 g of the above boehmite sol were added to a mixture of 118.17 g ofGPTS and 62.50 g of tetraethoxysilane (TEOS). The reaction mixture wasstirred at room temperature for 2 h and then 18.93 g of aluminiumtributoxyethoxide were added with ice cooling. The resulting sol wasstirred at room temperature for 2 h and then 93.14 g of the aboveboehmite sol and 79.30 g of butoxyethanol were added with ice cooling.The pot life was several months on storage at 4° C.

c) Production of the Coating

The coating sol was applied by spin coating to powder-coated metalpanels. The viscosity of the material was adapted to the process byadding 1-butanol. The panels were flashed off at 25° C. for 5 minutesand cured at 160° C. for 30 minutes.

d) Characterization

A transparent coating of 6 μm in thickness, containing cracks, wasobtained on the powder-coated surface. The other properties aresummarized in Table 1 column 3.

EXAMPLE 2

a) Preparation of a Boehmite Sol

104.62 g of 0.1 N HCl were added to 12.82 g of acetic acid-stabilized(6.4% by weight acetic acid) boehmite powder. Subsequent ultrasoundtreatment (20 minutes) produced a transparent, colourless solution whichwas used further directly to prepare the coating sol.

b) Preparation of the Coating Sol

24.3 g of the above boehmite sol were added to a mixture of 118.17 g ofGPTS and 62.50 g of TEOS. The reaction mixture was stirred at roomtemperature for 2 h and then 18.93 g of aluminium tributoxyethoxide wereadded with ice cooling. The resulting sol was stirred at roomtemperature for 2 h and then 93.14 g of the above boehmite sol and 79.30g of butoxyethanol were added with ice cooling. The pot life was severalmonths on storage at 4° C.

c) Production of the Coating

80.0 g of 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate weredissolved with stirring in 396.34 g of the coating sol. The coating solwas applied by spin coating to powder-coated metal panels. The viscosityof the material was adapted to the process by adding 1-butanol. Thepanels were flashed off at 25° C. for 5 minutes and cured at 160° C. for30 minutes.

d) Characterization

A crack-free transparent coating of 6 μm in thickness, was obtained onthe powder-coated surface. The other properties are summarized in Table1 column 4.

TABLE 1 Properties of uncoated and coated substrates Substrate SubstrateUncoated coated in coated in substrate accordance accordance (powdercoating with with Test surface) Example 1 Example 2 Appearance¹ nocracks cracks no cracks after curing Adhesion² 0/0 not 0/0 applicableowing to cracks Cyclic >12 not 5 temperature applicable stability³ owingto cracks Abrasion⁴ 18 not 4 applicable owing to cracks Acid no damagenot no damage stability⁵ applicable owing to cracks Baking oven nodamage not no damage spray⁶ applicable owing to cracks ¹Appearance:Visibility of cracks at magnifications 1x to 40x ²Adhesion: Cross-cut,tape test (0 = best score) ³Temperature cycling: 1 cycle = 10 min at200° C. then 10 min of cooling to RT; number of cycles until cracksbecome visible at magnifications 1x to 40x ⁴Abrasion: Taber Abraser,1000 cycles, 500 g load, CS 10F wheels, loss of mass in mg/1000 cycles⁵Acid stability: 30 min exposure to vinegar essence (brown) at RT and50° C., visual evaluation for damage to the coating ⁶Baking oven spray:30 min exposure to baking oven spray at RT and 50° C., visual evaluationfor damage to the coating

We claim:
 1. A powder-coated substrate having on a powder-coated surface thereof a topcoat comprising the cured product of: (a) a condensate based on at least one hydrolyzable silane containing at least one non-hydrolyzable substituent, the at least one hydrolyzable silane having an epoxide group on at least one of the at least one non-hydrolyzable substituents; (b) a curing catalyst selected from the group consisting of Lewis bases, titanium alkoxides, zirconium alkoxides, and aluminum alkoxides; (c) a nanoscale particulate inorganic solid having a particle size in the range from 10 to 100 nm; and; (d) at least one organic monomer, oligomer, or polymer, each containing at least one epoxide group.
 2. A powder-coated substrate of claim 1 where the curing catalyst is aluminum tri(butoxyethoxide), aluminum tri(sec-butoxide), or a mixture thereof.
 3. A powder-coated substrate of claim 1 where the at least one hydrolyzable silane having an epoxide group is a compound of the formula, X₃SiR where each X is independently selected from hydrolyzable groups and R is a glycidyloxy-C₁₋₆ alkylene group.
 4. A powder-coated substrate of claim 3 where the at least one hydrolyzable silane having an epoxide group is glycidyloxypropyltrimethoxysilane, glycidyloxypropyltriethoxysilane, or a mixture thereof.
 5. A powder-coated substrate of claim 1 where the nanoscale particulate solid comprises nanoscale particles of at least one compound selected from the group consisting of the oxides, oxide hydrates, nitrides, and carbides of Si, Al, B, Zn, Cd, Ti, Zr, Ce, Sn, In, La, Fe, Cu, Ta, Nb, V, Mo, and W.
 6. A powder-coated substrate of claim 5 where the nanoscale particulate inorganic solid comprises nanoscale particles of boehmite.
 7. A powder-coated substrate of claim 1 where the condensate comprises a co-condensate of: (a) at least one hydrolyzable silane having an epoxide group; and (b) at least one other hydrolyzable compound of an element selected from the group consisting of Si, Ti, Zr, Al, B, Sn, and V, the amount of the at least one other hydrolyzable compound being not more than 80 mol % of the total of the at least one hydrolyzable silane having an epoxide group and the at least one other hydrolyzable compound.
 8. A method of making the powder-coated substrate of claim 1 comprising: (i) applying to a powder-coated surface of a powder-coated substrate a topcoat comprising: (a) a condensate based on at least one hydrolyzable silane containing at least one non-hydrolyzable substituent, the at least one hydrolyzable silane having an epoxide group on at least one of the at least one non-hydrolyzable substituents; (b) a curing catalyst selected from the group consisting of Lewis bases, titanium alkoxides, zirconium alkoxides, and aluminum alkoxides; (c) a nanoscale particulate inorganic solid having a particle size in the range from 1 to 100 nm; and; (d) at least one organic monomer, oligomer, or polymer, each containing at least one epoxide group; and (ii) curing the topcoat. 